1. Field of the Invention:
This invention relates to a focus detecting device adapted for a photographic camera, a video camera or the like and more particularly to a focus detecting device arranged to have the pupil of an image forming lens into two regions, to form two secondary object images by light fluxes passing through these regions and to detect the focal position of the image-forming lens by detecting the relative positional relation between these secondary object images.
2. Description of the Related Art:
The conventional relatively accurate passive type focus detecting methods include a method called the image displacement method. An example of this method was proposed in Japanese Laid-Open Patent Appication No. SHO 55-155331. The optical system disclosed in that publication is as follows: Referring to FIG. 1 of accompanying drawings, the illustration includes an image-forming lens 1 which is something like the objective lens of a camera; an exit pupil 11 of the image-forming lens 1; a predetermined imaging plane 2 for the image-forming lens; a field lens 3; a pair of stops 8a and 8b symmetrically arranged relative to the optical axis X of the image-forming lens 1; a pair of re-imaging lenses 4 and 5 which are also symmetrically arranged relative to the optical axis X in the rear of the stops 8a and 8b; and light receiving devices 6 and 7 disposed near to the imaging planes of the re-imaging lenses 4 and 5. Each of the devices 6 and 7 consists of a plurality of light receiving photosensitive elements. The pair of light receiving devices may be replaced with one long piece of such a device. The field lens 3 is arranged to have the stops 8a and 8b in a conjugate relation to the exit pupil 11. This arrangement substantially divides the exit pupil 11 into two regions 1a and 1b. A light flux passing through the region 1a of the exit pupil 11 of the image-forming lens 1 forms a primary object image near to the predetermined imaging plane 2. After this, the light flux is caused through the field lens 3 by the re-imaging lens 4 to form a secondary object image on the surface of the light receiving device 6. Meanwhile, another light flux which passes through the opposite region 1b of the exit pupil 11 of the image-forming lens 1 likewise forms another secondary object image on the surface of the other light receiving device 7.
The position in which a primary object image is formed by the image-forming lens 1 (or a defocus degree) relative to the position of the secondary object images is univocally determined. In the image displacement method, the focus detecting operation is carried out by detecting the positional relation between the two secondary object images. Therefore, the secondary object image forming performance on the surfaces of the two light receiving devices 6 and 7 is greatly dependent upon focus detecting accuracy. Assuming that an on-axial point on an object O is P and an off-axial point on the object is Q, when a primary object image is formed on the predetermined imaging plane 2, the two points P and Q become P' and Q'. Next, with the primary image re-imaged by the reimaging lenses 4 and 5 on the light receiving devices 6 and 7, the points P' becomes p" and P"' while the other point Q' becomes Q" and Q"' respectively. Considering now the point Q', while the point Q' is located straight in front of the lens 5, it is located obliquely in front of the other lens 4. In other words, in photographing the point Q', the photo-taking angle of view of the lens 4 differs from that of the lens 5 thus diving different imaging conditions. More specifically, due to the influence of the distortion, chromatic aberration, etc. of the re-imaging lenses 4 and 5, a distance Z1 between the points P" and Q" and a distance Z2 between the points P"' and Q"' do not coincide with each other even when the image-forming lens 1 is in an in-focus state. The same problem also exists with respect to other points on the object O such as a point R, for example. A distance Z3 between points R" and P" and a distance Z4 between points R"' and P"' on the light receiving devices 6 and 7 do not coincide with each other. In the case of FIG. 1, distances Z1 and Z4 are equal and distances Z2 and Z3 also equal to each other.
In the conventional focus detecting method described, two images are arranged to electrically overlap each other by correlating signals relative to images of points Q" and R" with signals relative to images of points Q"' and R"'. However, with the uneven distortions arising in the images as mentioned above, the two images fail to precisely coincide with each other. In other words, while the point P" should coincide with the point P"' with the point Q" coinciding with the point Q"', the condition of Z1 .noteq.Z2 prevents the points P" and P"' from coinciding with each other. This has been lowering the accuracy of focus detection.